1. Field of the Invention
The present invention relates to electrically excited lasers and more particularly concerns improved methods and apparatus for accomplishing one or both of stabilization and frequency modulation of an electrically excited laser.
2. Description of Related Art
Variation of laser frequency is important in applications such as spectroscopy, optical communications, and laser radar. Frequency stabilization also is important in many applications. Frequency stabilization of a laser is required because frequency will change or drift for a number of reasons, primarily those which change the laser temperature. As the laser temperature changes, its effective cavity length changes, thereby changing cavity resonant frequency. In the past, frequency control for either stabilization or frequency modulation has been accomplished by several different techniques. For example, one of the laser cavity mirrors has been mounted to a piezoelectric transducer which has been energized with a frequency modulating signal or with a stabilizing feedback signal so as to change the length of the resonant cavity of the laser, and thus change its frequency. However, the piezoelectric transducer exhibits non-linear and hysteresis effects which degrade its frequency control or stabilization.
Another form of frequency modulation (frequency switching) for frequency chirp laser radar is described in an article entitled "Frequency-switchable CO.sub.2 laser: design and performance" by R. L. Shoemaker, R. E. Scotti, B. Comaskey, and Jose Soto M. in Applied Optics, Mar. 1, 1982/Vol. 21, No. 5, pages 961 through 966.
In the system described in the article of Shoemaker, et al, an electro-optic modulator crystal, such as a cadmium teluride (CdTe) crystal, is mounted inside the laser cavity and subjected to a high voltage which produces rapid switching of laser output frequency. A scheme similar to that of the Shoemaker et al article is described in U.S. Pat. Nos. 4,666,295 for Linear FM Chirp Laser of Robert L. Duvall III, et al, and 4,660,206 for Chirp Laser Stabilization System of Halmos et al, both assigned to the assignee of the present application. These patents describe laser radar transmitters.
Other modulation techniques proposed include intracavity rotating spiral ramp, a translating linear grating and a rotating circular grating. However, such techniques have not been successfully implemented insofar as the applicants are aware. In any event, even if such techniques are implemented, they would involve exceedingly expensive components and manufacturing techniques, and high complexity. Thus, present known techniques for laser frequency control are costly, complex and inefficient. It is an object of the present invention to provide laser frequency control, stabilization, or both, that is effective, efficient, relatively simple, and relatively inexpensive.